The positioning and deployment of medical appliances within a target site of a patient is a common, often-repeated procedure of contemporary medicine. These appliances or implants are used for innumerable medical purposes including the reinforcement of recently re-enlarged lumens and the replacement of ruptured vessels.
Coatings are often applied to these medical appliances to increase their effectiveness. These coatings may provide a number of benefits including reducing the trauma suffered during the insertion procedure, facilitating the acceptance of the medical appliance into the target site, and improving the post-procedure effectiveness of the appliance.
Expandable stents, stent grafts, balloon delivery systems, and aneurism coils are specific examples of medical appliances or implants that may be coated and inserted within the body. Expandable stents are tube-like medical appliances that often have a mesh-like structure designed to support the inner walls of a lumen. These stents are typically positioned within a lumen and, then, expanded to provide internal support for it. Because of the direct contact of the stent with the inner walls of the lumen, stents have been coated with various compounds and therapeutics to enhance their effectiveness. When this coating is haphazardly applied or has somehow been removed during the stent's manufacture or delivery, the stent's effectiveness can be compromised. In certain circumstances, defective implanted stents must be removed and replaced through a second medical procedure—an unwanted result.
Indiscriminate coating methods such as dip-coating and spray-coating have been used to coat stents as well as other medical appliances. These methods are, however, both wasteful and difficult to control. For example, dipping can result in non-uniform application of the coating to the appliance, thereby placing more coating at one end or region of the stent and making it difficult to predict the dosage of therapeutic that will be delivered when the stent or other appliance is implanted. The indiscriminate nature of dipping is also problematic as it may lead to the cracking and crumbling of coating at the junctions, hinges, and flexing members of the mesh-like stents. The coating that covers the hinged portions of the stent is highly susceptible to exfoliation because, as the stent is expanded, intolerable stresses may develop within the coating.
FIGS. 1 and 2 are illustrative of some of the concerns stemming from an indiscriminate coating process like dipping. In FIG. 1, stent 11 is shown in a closed, pre-deployment state. Here, stent 11 has been previously dipped in a vat of therapeutic in the direction of arrow 16. In other words, the right side of stent 11 was the leading edge entering the dipping vat. As can be seen, the coating of stent 11 is heavier on the right side than on the left side and covers each of the junctions 13 throughout the entire stent 11. As can also be seen, the coating becomes progressively thicker and covers more of the space between each of struts 12 as you travel from the left side to the right side of stent 11. This increasing thickness of coating is indicative of a stent that has been dipped and let stand on one of its ends as the coating dries and adheres to the stent.
FIG. 2 shows the unevenly coated stent 11 of FIG. 1 in an expanded state as it may be after it is positioned within a body. As is evident, the expansion of stent 11 has led to the cracking and crumbling of coating 15. Also evident is that the coating has been removed from most if not all of the junction points 13 after stent 11 has been expanded.